In one type of offshore oil and gas production, the christmas tree or production tree will be located at the subsea floor. One type of production tree, referred to as a horizontal tree, has a landing shoulder for a tubing hanger, and both the tubing hanger and the tree have lateral production flow passages that register.
While completing the well with a horizontal tree, a riser extends from the production tree to a floating vessel at the surface. Production tubing is attached to a tubing hanger and lowered from the vessel into the well during completion. A running tool secures to the tubing hanger, and a completion safety module, also commonly referred to as a subsea test tree forms a part of the running tool assembly. The assembly is lowered on a monobore conduit, such as drill pipe.
After the tubing hanger lands, the well is perforated by running a perforating gun through the tubing string. The subsea test tree has valves to open and close the monobore conduit for testing the well. After testing is completed, the valves of the subsea test tree are opened and a plug is lowered on a wireline through the monobore conduit and landed in the upper portion of the tubing hanger to block the vertical passage through the tubing hanger. The operator then detaches the running tool from the tubing hanger, retrieves the running tool, subsea test tree and monobore conduit to the surface. A tree cap then is lowered and landed in the bore of the production tree above the tubing hanger. The riser is disconnected from the tree. Production flows out the lateral flow passage.
It is possible for an emergency to occur while the tubing is being installed in the well. For example, the valves in the subsea test tree may malfunction and not be able to open. The downhole safety valve, which is a valve located in the tubing string below the tubing hanger, may be leaking or mechanically prevented from closure by objects in the tubing. Under such an emergency, the operator will likely need to kill the well, which is to load the tubing with a heavy enough fluid such that no pressure will exist at the surface. However, it may not be possible to pump directly down the monobore conduit because ofthe malfunctioning subsea test tree valves. In the prior art, subsea test trees have been employed that utilize valves that allow the operator to pump down the monobore conduit past the valves even though closed. These types of valves are considered to have potential have drawbacks as to reliability, however, due to a greater possibility of particulate ingress between seals and mating surfaces.